necessitate either expensive and troublesome submarine cables and conduits or special towers for carrying an overhead span.

Ninth. The inherent simplicity of the vertical lift as a piece of mechanism, compared with the bascule, makes it more reliable in operation, and, on that account, somewhat less expensive. For this reason the vertical lift would have an advantage over the baseule in many foreign countries, such as those of Latin America, where the conveniences for repairing or replacing parts are not close at hand.

Erection requirements or other special conditions at a site often affect materially the relative economics of the various types of movable spans—sometimes to such an extent as to outweigh all other considerations. They may affect either favorably or unfavorably any of the different types. Any given site should, therefore, receive special study from this viewpoint.

Economics of Swing Spans

Although the author does not believe that there is to-day any necessity for this type of structure nor any advantage to be derived from building one; yet, as all engineers may not agree with him, it will be well, in order that this chapter may not be lacking in completeness, to give a short dissertation concerning the economics of some of the different types of swing in common use.

Rim-Bearing versus Center-Bearing Spans

The choice between these two types is mainly a matter of taste or sometimes one of prejudice; for there is no great difference in their first costs, what there is being in favor of the latter, which also has a slight advantage in respect to amount of power required to operate. In the author's opinion, the principal economic advantage of the center-bearing type is due to the smaller diameter of the pivot pier.

There is a difference of opinion amongst railroad engineers, and even amongst high authorities on bridges, concerning both the relative merits and the economics of these two types. The late C. C. Schneider, Past President of the American Society of Civil Engineers, said: "The center-bearing type, designed in accordance with good modern practice, offers more advantages than the rim-bearing type, and should always receive the first consideration in determining upon a design. It requires less power to turn, has a smaller number of moving parts, is less expensive to construct and maintain, involves less accurate construction than the rim-bearing bridge, and does not as easily get out of order. The structural and the operating-machinery parts are entirely separate; and when the bridge is closed, it forms either two independent fixed spans, or a fixed span, continuous over two openings, resting on firm, substantial supports.